[SI-LIST] Re: Can a thin PCB trace be used as a reliable fuse

Yes, you can use a thin trace as  fuse. How do you fix the thing when the
"fuse" goes? Do you build a new pcb and populate it again? The whole point
of a fuse is that you change the fuse if there is a fault. You don't throw
away the PCB.

Cheers,


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Hofmann, Mark
Sent: Wednesday, August 25, 2004 10:59 AM
To: 'a.ingraham@xxxxxxxx'; 'Si-List'
Subject: [SI-LIST] Re: Can a thin PCB trace be used as a reliable fuse


Well, I hate to say this, but this very thing was done at a company that
makes fire alarm panels.  If UL allowed that, they'll probably allow it in
your application as well.  Just remember that the i2t value of the trace can
vary WIDELY.  How wide?  I think it was on the order of +/- 50%.

Mark Hofmann

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Andrew Ingraham
Sent: Wednesday, August 25, 2004 9:47 AM
To: Si-List
Subject: [SI-LIST] Re: Can a thin PCB trace be used as a reliable fuse

I don't know the regulatory details here ...

... but I'd be very concerned about things like fire.  When a trace blows,
is there a flame?  In a packaged fuse, it would be contained within the
package.  Not so with an open trace.

Even if no flame, does something melt, and could it fall somewhere else?

Are toxic materials released, due to the uncontained nature of your
makeshift fuse?

I would not think that a thin board trace would make a very reliable fuse
with good repeatability, but that's not much more than a guess.  External
layers have loose tolerances on trace thickness, because of the way they are
made and because it has minimal effect on Zo.

Also I'd think that the heat sinking properties of the board might make it a
rather slow acting fuse, and in the meantime a lot of the board "cooks" as
well as all nearby components (you did say you are tight on space).

Even attaching short lengths of nichrome wire might be better (if nichrome
makes a good fuse)...

Regards,
Andy


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